1. Field of the Invention
The present invention relates to systems and methods for transmitting satellite signals, and in particular to a system and method for reconfigurably transmitting shaped beam satellite signals via reflector array antennas.
2. Description of the Related Art
Communications satellites are in widespread use. The communications satellites are used to deliver television and communications signals around the Earth for public, private, and military uses.
The primary design constraints for communications satellites are antenna beam coverage and radiated Radio Frequency (RF) power. These two design constraints are typically thought of to be paramount in the satellite design because they determine which customers on the Earth will be able to receive satellite communications service. Further, the satellite weight becomes a factor, because launch vehicles are limited as to how much weight can be placed into orbit.
Many satellites operate over fixed coverage regions, such as the Continental United States (CONUS), and employ polarization techniques, e.g. horizontal and vertical polarized signals, to increase the number of signals that the satellite can transmit and receive. These polarization techniques use overlapping reflectors where the reflector surfaces are independently shaped to produce substantially congruent coverage regions for the polarized signals. This approach is limited because the coverage regions are fixed and cannot be changed on-orbit, and the cross-polarization isolation for wider coverage regions is limited to the point that many satellite signal transmission requirements cannot increase their coverage regions.
Many satellite systems would be more efficient if they contained antennas with an on-orbit reconfigurable beam, capable of modifying the shape and translation (or scan) of the beam on the Earth. These objectives are typically met using a multi-feed reflector antenna system that reconfigures the beam coverage by individually varying signal amplitude with variable attenuators or amplifiers and varying the signal phase with variable phase shifters at the feed elements located along the reflector focal plane.
However, the antenna feed system and beamforming network (BFN) of such prior art multi-feed reflector antennas is complex, lossy, heavy, difficult to integrate, test, and repair or replace, requiring excessive time and labor costs. Furthermore, the complexity of the antenna feed system of such prior art multi-feed reflector antenna systems makes them more difficult to operate. Particularly, the amplifiers of prior art multi-feed reflector antenna systems do not operate at a fixed power level when reconfiguring the beam coverage. In addition, reconfiguring the beam coverage of prior art multi-feed reflector antenna systems requires switching power among a plurality of feeds.
Another approach to meet the previous beam reconfigurability objectives is to use a Direct Radiating Array (DRA). In the DRA solution, no reflector is used. The feed elements are arranged in a grid pattern and pointed directly at the coverage area. The antenna beam phase can be reconfigured by varying the excitation phase at the feed elements with variable phase shifters. The disadvantage of this solution is that to obtain the same directivity as a reflector antenna, a very large number of feed elements and phase shifter are needed, making such an antenna system very heavy and complex.
There is therefore a need in the art for a reconfigurable multi-feed reflector antenna system without the attendant complexity of prior art systems. There is also a need in the art for a reconfigurable multi-feed reflector antenna system that is easier to integrate and test. There is a further need in the art for a reconfigurable multi-feed reflector antenna system using amplifiers operating at a fixed gain. There is yet another need in the art for a reconfigurable multi-feed reflector antenna system that is reconfigured without switching power among a plurality of feeds.
The present invention satisfies these needs.
To address the requirements described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses an apparatus and method for transmitting signals with a phase-only reconfigurable multi-feed reflector antenna. The present invention further discloses a feed network.
In general, the reconfigurable multi-feed reflector antenna system of the present invention is achieved by employing a less complicated approach to reshape and scan the beam of a multi-feed reflector by varying only the relative excitation phase at each feed element, while maintaining a fixed signal gain at the feed elements. The phase of each element can be controlled using ordinary variable phase shifters. In addition, the system and method of the present invention may be implemented with a reflector gimbal mechanism to further extend beam coverage translations.
A reconfigurable multi-feed antenna system in accordance with the present invention comprises a reflector for reflecting RF signals having a reflector plane and a feed array comprising a plurality of feed elements wherein said feed array is defocused from said reflector focal plane, yet produces an RF wavefront substantially similar to an RF wavefront that would be produced by a feed array located at the reflector focal plane.
A feed network in accordance with the present invention comprises a BFN comprising a signal divider for dividing an input signal into a plurality of divided signals, a plurality of variable phase adjusters, each receiving one of the plurality of divided signals and outputting a phase adjusted signal, and at least one fixed gain amplifier for amplifying each phase adjusted signal and outputting an amplified signal for each phase adjusted signal. The feed network further comprises a feed array, defocused from a reflector, comprising a plurality of feed elements, each receiving an amplified signal and radiating a radiated signal, wherein the combination of the radiated signals forms a wavefront.
A method of transmitting a signal in accordance with the present invention comprises forming an RF wavefront with a feed array, wherein said feed array is defocused from a reflector focal plane, yet produces an RF wavefront substantially similar to an RF wavefront that would be produced by a feed array located at the reflector focal plane and reflecting said wavefront to a coverage area.
The foregoing allows the use of a constant value for the gain at each feed element which in turn enables three fundamental advantages of the present invention. First, the present invention provides the advantage that the amplifiers feeding the elements have a fixed operating power level, regardless of the coverage shape. Second, reconfiguring the beam coverage does not require switching power among feeds. Third, the overall antenna feed system is less complex and simpler to control than prior art systems.